1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers, and in particular, to a dielectric filter and a dielectric duplexer for use in a microwave or milliwave-band communication apparatus or the like.
2. Description of the Related Art
As shown in FIG. 14, a known conventional dielectric filter 30 is formed by connecting in series transverse electric (TE).sub.10 -mode dielectric resonators 31a and 31b. Conductors 33a and 33b are formed on substantially the entire surfaces of rectangular parallelepiped dielectric bodies 36a and 36b, except that on the connection surfaces of the resonators 31a and 31b, rectangular coupling windows 32a and 32b having no the conductors are respectively formed. Connecting the resonators 31a and 31b causes the coupling windows 32a and 32b to correspond to each other, and the resonators 31a and 31b are inductively or capacitively coupled via the coupling windows 32a and 32b. In this arrangement, the connection surfaces of the resonators 31a and 31b are not exposed to the outside of the dielectric filter 30. At ends of the resonators 31a and 31b are formed input/output electrodes 34a and 34b which are isolated from the conductors 33a and 33b. The resonators 31a and 31b are also provided with external coupling holes 35a and 35b. The central frequency of the dielectric filter 30 is determined by dimensions in the length direction (x direction denoted by an arrow in FIG. 12) and the width direction (y direction denoted by an arrow in FIG. 12) of the resonators 31a and 31b. In addition, other characteristics (the electromagnetic coupling amount, pass-band width, etc. between the resonators 31a and 31b) of the dielectric filter 30 are determined by the sizes of the coupling windows 32a and 32b and so forth.
In general, when the characteristics of a dielectric filter are to be adjusted, characteristics such as the pass-band width are measured, with a plurality of resonators connected to one another, and then the adjustment is performed by changing the sizes of the coupling windows 32a and 32b and so forth in accordance with the measured results. However, with the conventional dielectric filter 30, when the resonators 31a and 31b are connected to each other, the coupling windows 32a and 32b are not exposed at the outer surfaces of the dielectric filter 30. Accordingly, in order to expose the connection surfaces of the resonators 31a and 31b on which the coupling windows 32a and 32b are formed, it is necessary to separate the resonators 31a and 31b. In addition, after performing an adjustment by using a cutting tool like a router to cut the conductors 33a and 33b peripheral to the coupling windows 32a and 32b to change the sizes of the coupling windows 32a and 32b and so forth, the resonators 31a and 31b must be connected again. Accordingly, the adjustment operation is complicated. Also, after changing the sizes of the coupling windows 32a and 32b and so forth, it is difficult to restore the connection between the resonators 31a and 31b with preferable reproducibility, which causes a problem in the stability of the characteristic adjustment.